Standard solid-phase immunoassays with monoclonal antibodies involve the formation of a complex between antibody adsorbed on a solid phase (capture antibody), antigen, and antibody to another epitope of the antigen conjugated to a detectable label, e.g. an enzyme (tracer antibody). Thus, a sandwich is formed: solid phase-capture antibody-antigen-tracer antibody. In the reaction catalyzed by the sandwich, the activity of the antibody-conjugated enzyme is proportional to the antigen concentration in the incubation medium. The standard sandwich method is also called double antigen bridging immunoassay because capture and tracer antibodies bind to different epitopes of the same antigen. Hoesel, W., et al., J. Immunol. Methods 294 (2004) 101-110, report an anti-EPO double antigen bridging assay whereby a mixture of immobilized rhEPO coupled to amino groups and to carbohydrate groups was used. Immunoassays such as the double antigen bridging ELISA are common assay types in the investigation of an immunogenic answer of a patient to an antibody drug. Mire-Sluis, A. R., et al., J. Immunol. Methods 289 (2004) 1-16, summarize the recommendations for the design and optimization of immunoassays using detection of host antibodies against biotechnology products. According to Mire-Sluis et al. the well-known anti-drug antibody assay formats show considerable disadvantages. Anti-drug antibody assays are mentioned, for example, in WO 2005/045058 and WO 90/006515. Anti-idiotypic antibody assays are mentioned, for example, in U.S. Pat. No. 5,219,730, WO 87/002778, EP 0 139 389, and EP 0 170 302. Wadhwa, M., et al., J. Immunol. Methods 278 (2003) 1-17, report strategies for the detection, measurement and characterization of unwanted antibodies induced by therapeutic biologicals. The principles of different immunoassays are described, for example, by Hage, D. S., Anal. Chem. 71 (1999) 294R-304R. Lu, B., et al., Analyst. 121 (1996) 29R-32R, report the orientated immobilization of antibodies for the use in immunoassays. Avidin-biotin-mediated immunoassays are reported, for example, by Wilchek, M., and Bayer, E. A., Methods Enzymol. 184 (1990) 467-469. A comparison of ELISA and surface plasmon resonance was reported by Lofgren, J. A., et al., J. Immunol. 178 (2007) 7467-7472.